1. Field of the Invention
This invention relates to a method and apparatus for measuring the radio frequency (RF) energy leakage of shielded enclosures.
2. Description of the Prior Art
In testing RF or other sensitive electronic equipment, it is often necessary to make tests within a room shielded against RF signals to prevent extraneous signals from interfering with the test measurements being made. In addition, RF shielded enclosures are provided as cabinets for RF transmitters and receivers. Such shielding is necessary in order to prevent radiation of extraneous RF signals generated within the cabinet in the case of electronic equipment which radiates electromagnetic energy and to prevent extraneous RF signals or noise from outside the enclosure from disrupting or interfering wiyh normal operation of sensitive electronic equipment. Shielded rooms are also necessary to prevent compromising emanations from data processing equipment which is processing information important to national security.
Seam Leak Detectors (SLDs), such as the one described in U.S. Pat. No. 3,492,568 to Johnson, have been commercially available for several years. These SLDs use two major components, an RF current source and a hand-held receiver with an antenna probe. To operate the SLD, the RF current source is connected to a pair of diagonally opposite corners of a shielding enclosure by means of long wires and large spring loaded clips.
The current from the RF current source spreads over the enclosure in a complex manner. At the seams, or other leakage points, electromagnetic fields are developed inside the enclosure either from leakage directly through slits or apertures or from voltage developed across the electrical resistance of the seams. In the case of thin shields, some RF energy may diffuse through the shield.
The fields inside the enclosure are detected by the hand-held receiver. It is generally recommended that the measurements be repeated while applying the RF current signal to a second pair of diagonally opposite corners. Typically, the SLD's receiver provides a meter reading in decibels relative to an arbitrary value with the receiver gain being adjustable and an attenuator provided to assure a wide dynamic range of measurement.
Such prior art SLDs suffer from inaccurate measurements due to the uneven current distribution from the SLD RF current sources over the shielded room surfaces or from electrical null effects that exists for each pair of diagonally opposite corners. In addition, the measurements are absolute because prior measurements made on the enclosure are not available for comparison with present measurements. Accordingly, it is impossible to measure the amount of degradation in or loss of shielding effectiveness resulting from use of the enclosure. Also, such devices do not provide a quantitative measurement of the effectiveness of a shield for attenuating RF energy that compares favorably with the methods described by MIL-STD-285.